Population Growth and Age Distribution: Exponential and Logistic Models - Prof. Kim Largen, Study notes of Environmental Science

Download Population Growth and Age Distribution: Exponential and Logistic Models - Prof. Kim Largen and more Study notes Environmental Science in PDF only on Docsity! 1 Populations: Population Ecology EVPP 110 Lecture Instructor: Dr. Largen Fall 2003 2 Population ecology 4Population – definition – major characteristics – dynamics – life histories 3 Population definition 4Population – definition • group of individuals of a species living in same area at same time – using common resources – regulated by same natural phenomena 4 Figure: Monarch butterflies 5 Population definition 4Population – definition • flexible • allows discourse in similar terms about any population – – – 6 Figure: Aerial census for African buffalo (Syncerus caffer) in the Serengeti of East Africa 7 Population characteristics 4Populations – major characteristics • size • density • dispersion • age distribution 8 Population characteristics 4 Population size – definition • number of individuals – important feature of any population 9 Population characteristics 4 Population size – affects ability of population to survive • small populations tend to become extinct – endangered by random events – inbreeding » » 10 Population characteristics 4Population density – definition • number of individuals in a certain area or volume – # trees per km2 of forest – # earthworms per m3 of soil 11 Population characteristics 4Population density – important to survival of population • individuals spaced widely apart may rarely encounter one another – limits reproductive capacities » 12 Population characteristics 4Population density – how is population density measured? • impossible or impractical to count all individuals in a population – – • use sampling techniques 13 Population characteristics 4 Population density – sampling technique • method to estimate population density – direct count of organisms or indicators in small area or volume » used to project actual density over entire area or volume – examples » » 14 Population characteristics • depending on interactions between – biotic potential » growth factors – environmental resistance » decrease factors 32 Factors that affect size of population 4Factors that affect size of population – biotic potential • “growth factors” • capacity of a population for growth • varies – between populations – within population over time 33 Factors that affect size of population 4 Factors that affect size of population – biotic potential • factors that favor increase in size – abiotic » favorable light » favorable temperature » favorable chemical environment (optimal level of critical nutrients) 34 Factors that affect size of population 4 Factors that affect size of population – biotic potential • factors that favor increase in size – biotic (such as) » high reproductive rate » generalist » adequate food » adequate defenses from predators » resistance to diseases 35 Factors that affect size of population 4Factors that affect size of population – environmental resistance • “decrease factors” • all the factors acting jointly to limit growth of a population 36 Factors that affect size of population 4 Factors that affect size of population – environmental resistance • factors that lead to decrease in size – abiotic » too much, too little light » temperature too high, too low » unfavorable chemical environment (critical nutrients too high, too low) 37 Factors that affect size of population 4 Factors that affect size of population – environmental resistance • factors that lead to decrease in size – biotic (such as) » low reproductive rate » specialist » inadequate food » inadequate defenses from predators » inability to resist diseases 38 Factors that affect size of population 4Factors that affect size of population – biotic potential & environmental resistance • together determine – carrying capacity (K) » number of individuals of a given species that can be sustained indefinitely in a given area or volume 39 Types of population growth 4 Two types of population growth – exponential • accelerating increase in population size – occurs when growth is unregulated – logistic • population growth that is slowed by population-limiting factors – tends to level off at a carrying capacity 40 Types of population growth 4population growth – two types • exponential • logistic 41 Types of population growth 4 Exponential growth – exhibited by a population that has few, if any, resource limitations – starts out slowly, speeds up as population increases – rate of expansion that occurs under ideal conditions – entire population multiplies by a constant factor during constant time intervals 42 Types of population growth 4Exponential growth – described by equation G = rN • G = growth rate of the population • N = population size • r = intrinsic rate of increase – graph produces typical J-shaped curve 43 Types of population growth 4 Exponential growth – r = intrinsic rate of increase • rate at which a population would grow if it had unlimited resources – remains constant for any population expanding without limits • based on organism’s inherent capacity to reproduce – varies by organism 44 Types of population growth 4Exponential growth – r = intrinsic rate of increase • can be roughly estimated as – birth rate minus death rate – r = b - d 45 Types of population growth – occur in nature, over time • four general types exist – stable – irruptive – irregular – cyclic – most are poorly or incompletely understood 61 Population fluctuations 4Population fluctuations – stable • population size fluctuates around carrying capacity – slightly above – slightly below • typical of species in undisturbed tropical rainforests – little variation in average temperature or rainfall 62 Population fluctuations 4 Population fluctuations – irruptive • population is normally fairly stable • occasionally explodes (irrupts) to peak – then crashes to » stable lower level » very low level – due to factor (ie temp) that temporarily increases carrying capacity • examples: raccoon, house mouse 63 Population fluctuations 4Population fluctuations – irregular • irregular, chaotic behavior in population size – no apparent recurring pattern • may be due to – chaos in system – poorly understood interactions 64 Figure: Irregular population fluctuations 65 Population fluctuations 4Population fluctuations – cyclic • fluctuations in size that occur over a regular time period • most are poorly understood • include predator-prey cycles 66 Population fluctuations 4 Population fluctuations – predator-prey cycles • seen in some groups of species that interact as predator and prey – characterized by » sharp increases in numbers followed by » seemingly periodic crashes – classic example » snowshoe hare, Canadian lynx 67 Figure: snowshoe hare and lynx 68 Population fluctuations 4Population fluctuations – predator-prey cycles • explained by two hypotheses – top-down control – bottom-up control 69 Population fluctuations 4 Population fluctuations – predator-prey cycles • top-down control hypothesis – lynx prey on hare – reduces hare population – fewer hares support fewer lynxes – causes periodic reduction in lynx population » lag-time, offset from hare reduction 70 Population fluctuations 4 Population fluctuations – predator-prey cycles • top-down control hypothesis cont – reduced numbers of predators (lynx) allows population of prey (hare) to recover and increase – increased numbers of prey (hare) support increased numbers of predators and lynx population increases – cycle continues 71 Population fluctuations 4 Population fluctuations – predator-prey cycles • top-down control hypothesis cont – doubt has been cast on this explanation » snowshoe hares have been found to exhibit similar 10-year “boom-or-bust” cycles on islands where lynx are absent – leading to 2nd hypothesis » bottom-up control 72 Population fluctuations 4 Population fluctuations – predator-prey cycles • bottom-up control hypothesis – rather than cycle being driven by predator at top » might be driven by food source of prey (hare) at bottom 73 Population fluctuations 4 Population fluctuations – predator-prey cycles • bottom-up control hypothesis cont – reduction in quantity or quality of food source (plants) of hare leads to crash of hare population – fewer hare support fewer predators and lynx population crashes – reduction in hare population gives plant population time to recover 74 Population fluctuations 4 Population fluctuations – predator-prey cycles • bottom-up control hypothesis cont – increased plant population supports more hares and hare population increases – increased hare population supports more lynx and lynx population increases – cycle continues, driven by plant availability 75 Population fluctuations 4 Population fluctuations – predator-prey cycles • genuine examples of both top-down and bottom-up control exist in nature 76 Figure : Population cycles of the snowshoe hare and lynx 77 Survivorship and Life History Strategies 78 Survivorship and life history strategies 4Survivorship and life history strategies – survivorship • life tables • survivorship curves – life history strategies • opportunisitc life history • equilibrial life history 79 Survivorship 4 Survivorship – percentage of an original population that survives to a given age • requires compilation of data (life table) – for each defined age interval » number living at start of interval – reproduce when young – produce many offspring – provide little to no parental care of offspring – most offspring die before reaching reproductive age 94 Life History Strategies 4 Opportunistic(r-selected) life history – characteristics • populations – tends to grow exponentially » thus the name r-selected » due to high intrinsic rate of growth – live in unpredictable environments – controlled by density-independent factors – exhibit type III survivorship curve 95 Life History Strategies 4 Opportunistic(r-selected) life history – examples • bacteria • algae • most annual plants – dandelions • most insects – cockroaches • rodents • oysters 96 97 98 Life History Strategies 4equilibrial (K-selected) life history – put fairly little energy into reproduction • put most energy into long term survival – for purpose of being able to put lots of energy into nurturing and protecting offspring – are good competitors 99 Life History Strategies 4Equilibrial (K-selected) life history – are not considered opportunistic • thrive best in ecosystems with fairly constant environmental conditions – populations remain close to carrying capacity (K) over long periods of time 100 Life History Strategies 4 equilibrial (K-selected) life history – characteristics • organisms – larger-bodied – reproduce later in life – produce fewer offspring – provide high parental care – most offspring survive to reproductive age 101 Life History Strategies 4 Equilibrial (K-selected) life history – characteristics • populations – size tends to be stable » thus the name K-selected » populations tends to stay near carrying capacity (K) – live in predictable environments – controlled by density-dependent factors – exhibit type I survivorship curve 102 Life History Strategies 4Equilibrial (K-selected) life history – examples • humans • large trees • polar bears • elephants 103 104 Life History Strategies 4Intermediate life history – many organisms have life histories that fall between opportunistic and equilibrial • exhibit type II survivorship curve • examples – many birds – squirrels – hydra 105 The End.